Currently, toxicological risk assessment lacks direct estimation of the magnitude and variability of human responses to environmental toxicants. Specifically, there is very little data on the effects of low dose toxicants on human developmental processes, including cellular differentiation. That is because the current basis of toxicological assessment, animals and immortalized cell lines, cannot adequately model the impact of low level toxicant exposure on human population. A potential alternative detection platform should enable incorporation of human variability in screening of a large number of toxicants in a high-throughput manner. To establish such a platform, we suggest using human donor- specific cells that can be (a) isolated in a non-invasive manner, (b) easily expanded in culture, and (c) cryopreserved without loss of viability. We propose that human endothelial progenitor cells or endothelial colony forming cells (ECFCs), a population of CD31+/CD34+ pluripotent cells found in circulation, would fulfill these requirements. In vitro, ECFCs are highly proliferative and can differentiate into mature endothelial cells (ECs). The overall goal of this research is to evaluate the effect of chemical toxicants on viability, proliferation, and differentiation of ECFCs derived from different individuals. Specifically in this proposal, we will determine the feasibility of ECFCs as a platform for high-throughput chemical toxicity testing. For this purpose, we selected three known environmental toxicants found at low levels in neonatal and adult peripheral blood: bisphenol A (BPA), perfluorooctanoic acid (PFOA), and cadmium. The proposed study is the key step in the development of a novel, robust, and comprehensive platform utilizing donor-specific pluripotent cells. At the conclusion of this project, we will demonstrate the amenability of ECFC-based assays for high- throughput evaluation of environmental toxicants. ECFC-based toxicological assays can become an invaluable tool for functional assessment of human genetic diversity and identification of population subgroups most vulnerable to chemical hazards.